The invention relates to a magnetooptical waveguide device for the conversion of modes of propagation of the device. The device comprises layers which are epitaxially applied to a substrate layer and of which at least the waveguide layer in which the modes propagate consists of a transparent magnetooptical material that can be acted upon by a magnetic field for the mode conversion. Further means for matching the phase velocities of the TE and of the TM modes are provided.
Such planar monomode waveguide structures required, for example, for optical isolators or circulators are known from GB PS No. 1,529,374.
In the waveguide layer, transverse electrical (TE) and transverse magnetic (TM) modes are converted into each other in a nonreciprocal manner (mode conversion). This is attained, for example, by the Faraday effect, which leads to TE/TM mode coupling in the waveguide layer magnetized in the direction of propagation. However, with this coupling the desired complete mode conversion is obtained only if both participating modes, i.e. the TE and TM modes, have the same speed of propagation (phase velocity) because only then a continuous and cumulative transmission of the energy from one mode to the other is possible.
For the difference .DELTA..beta. of the propagation constants .beta..sub.TM and .beta..sub.TE of the TE and TM modes, respectively, the value zero is therefore aimed at for phase matching.
For many applications in the optical communication technology, an extremely high efficiency of the non-reciprocal mode conversion in magnetooptical waveguides is desired, for example for isolators n.gtoreq.99.999%. This means especially for the practically important case of weakly propagating modes an extremely accurate phase matching. The extraordinarily exact adjustment of the refractive indices effective for both modes required to this end is not attainable with tolerable requirements with respect to the manufacturing accuracy.
In GB PS No. 1,529,374, it suggested to provide phase matching by means of additional layers on the waveguide layer. The required geometrically accurate disposition of these layers can be obtained only with difficulty.
From the Jap. J. Appl. Phys. 22 (1983), pages L618 to L620, it is known that the anisotropy obtained during the growth of the layers can be influenced in such a manner that the difference .DELTA..beta. becomes zero. For practical use, however, an extraordinarily high manufacturing accuracy would be required.
According to "Proc. 10.sup.th European Conference on Optical Communication 1984, page 42", a phase matching can also be obtained by a coating layer applied to the waveguide layer and having the form of a birefringent grating structure. This certainly also requires an undesirably high manufacturing accuracy.
Moreover, the known phase matching means have the great disadvantage that it is not possible to carry out corrections after the layers have been applied.
U.S. Pat. No. 4,220,395 describes mode coupling not in the waveguide layer, but at its interfaces with adjacent layers. Phase matching is obtained by means of a standing acoustic wave, whose acoustic frequency is adjusted so that .DELTA..beta.=0 is obtained. The arrangements to be provided for producing a standing acoustic wave are complicated and are not suitable to be successful in waveguide structures of the kind mentioned in the opening paragraph.